Semiconductor switches have been widely used for power converters in recent years, such as Insulated Gate Bipolar Transistor (IGBT), Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), SiC MOSFET and the like. However, a high percentage of the losses in the power converters come from the losses of the semiconductor switches.
The losses of the semiconductor switches include switching losses and conduction losses. The switching losses are related to the characters of driving circuits for the semiconductor switches and the switching frequency of the semiconductor switches. Therefore, different driving circuits using the semiconductor switches may cause different switching losses of the semiconductor switches.
Taking IGBT as an example, a common IGBT driving circuit is shown in FIG. 1. The driving circuit includes a first resistor R1, a first switching device Q1, a second switching device Q3 and a driving resistor Rd. After receiving a driving control signal DS, the first resistor R1 transmits the driving control signal DS to a totem-pole amplifier composed of the first switching device Q1 and the second switching device Q3. The amplified signal output from the totem-pole amplifier turns on or off the IGBT S by passing through the driving resistor Rd. FIGS. 2A and 2B show a relationship between the resistance of the driving resistor Rd and the turn-off loss Eoff of the IGBT S and a relationship between the resistance of the driving resistor Rd and the turn-on loss Eon of the IGBT S, respectively. It may be seen from the FIGS. 2A and 2B that with the decrease of the resistance of the driving resistor Rd, the switching speed is increased, and the turn-off loss Eoff and the turn-on loss Eon fall down accordingly. Therefore, smaller resistance of the driving resistor Rd may decrease the switching loss of the IGBT.
However, in actual circuit application, the resistance of the driving resistor Rd is limited by voltage stress. FIG. 3 shows a half-bridge circuit comprising IGBTs. An inverter system is formed by an LC filter, a first capacitor C1 and a second capacitor C2, which converts DC voltages across the first capacitor C1 and the second capacitor C2 connected in series at the DC side into an alternating current (AC) output voltage Vout. Ls1 and Ls2 in the figure are parasitic inductors in the main loop. When IGBT S1 is turned off, because of current continuity of the output inductor Lf, the current flowing through the IGBT S1 before is transferred to the current flowing through a diode D2 in anti-parallel with an IGBT S2 for freewheeling. During this process, voltages VLs1 and VLs2 are formed in the parasitic inductors Ls1 and Ls2 by the di/dt of the currents flowing through the IGBT S1 and the anti-parallelled diode D2, wherein VLs1=Ls1·di/dt, and VLs2=Ls2·di/dt. The voltage stress of the IGBT S1 at this moment is VS1=VC1+VC2+(Ls1+Ls2)·di/dt. A relationship between the voltage stress VS1 and the resistance of the driving resistor Rd is shown in FIG. 4. It can be seen from FIG. 4 that the smaller the resistance of the driving resistor Rd or the larger the current is, the larger the switching speed (didt) will be, thereby causing a larger voltage stress.
In FIG. 4, if the limit value of the voltage stress VS1 is 550V and the minimum turn-off current of the IGBT is 300 A, the minimum resistance of the driving resistor Rd may be 10Ω. However, it can be seen from FIGS. 2A and 2B that the Eoff and Eon when the resistance of the driving resistor Rd is 10Ω are much larger than the Eoff and Eon when the resistance of the driving resistor Rd is 2.5Ω, and thus the IGBT has a larger switching loss.
As mentioned above, there are contradictions between obtaining a smaller IGBT switching loss and obtaining a smaller IGBT voltage stress in a converter system. A prior scheme of a driving circuit is shown in FIG. 5. This circuit can make a driving resistor Rd as a turn-on resistor Rd1 for turning on the IGBT and a driving resistor Rd as a turn-off resistor Rd2 for turning off the IGBT. The driving circuit may obtain smaller turn-on resistance (Rd1/Rs2) with the larger turn-off resistance. However, even if the turn-on loss of the IGBT may be decreased by employing the circuit, the turn-off loss of the IGBT still cannot be decreased. Therefore, the system still has a higher switching loss.
Accordingly, a driving circuit and its method is highly required to overcome the foregoing defects.